Dye-sensitized solar cells (DSC's) developed by M Grätzel et al are a new type of solar cells made of low-cost materials and can be manufactured by conventional printing techniques, see for example U.S. Pat. No. 5,084,365.
A conventional sandwich type DSC is shown in FIG. 1. The DSC (1) has a few micrometer thick porous TiO2 electrode layer (2) deposited onto a transparent conducting substrate (3). The TiO2 electrode comprises interconnected TiO2 metal oxide particles dyed by adsorbing dye molecules (typically a Ruthenium polypyridyl complex) on the surface of the TiO2 particles. The transparent conducting substrate (3) is normally a transparent conducting oxide (TCO) (4), for example fluorine-doped tin oxide (FTO), deposited onto a glass substrate (5). Other types of TCO materials, such as indium tin oxide (ITO), or aluminum doped zinc oxide, or antimony doped tin oxide, are used as well.
The TCO layer (4) serves the function as a back contact extracting photo-generated electrons from the TiO2 electrode (2). The TiO2 electrode (2) is in contact with an electrolyte (6) (typically containing I−/I3− ion pairs) and another transparent conducting substrate, i.e., a counter electrode (7). The TCO layer (8) of the counter electrode is usually covered with a thin catalytic layer of platinum. The platinum has a strong catalytic effect, facilitating the electron transfer to the electrolyte.
Sunlight is harvested by the dye, producing photo-excited electrons that are injected into the conduction band of the TiO2 particles and further collected by the conducting substrate (8). At the same time, I− ions in the redox electrolyte reduce the oxidized dye and transport the generated electron acceptors species (I3−) to the counter electrode where the I3− species are reduced to I−. A record 11% power conversion efficiency has been reported, although good quality cells typically provide between 5% and 8%.
The edges of the conducting substrates are usually not deposited with TiO2 electrode material. The two conducting substrates are sealed at the edges in order to protect the DSC components against the surrounding atmosphere, and to prevent the evaporation or leakage of the DSC components inside the cell.
Due to the low conductivity of the transparent conducting oxide (4, 8), the cells (1) must be deposited in segments or strips with gaps in between. Current collectors are deposited in the gaps to connect the segments or strips to form solar cell modules. The wider the segments the greater the electronic ohmic losses in the TCO layer because of poor TCO conductivity.
The individual cells (1) are electrically connected in parallel or in series to enhance the DSC current or DSC voltage, respectively. The electrical connection can be made outside the cells using peripheral equipment such as cables or solders. Alternatively, the electrical connection can be made inside the cells by distributing the DSC components in such a way that the desired parallel or series connection of the cells is achieved.
The low conductivity of the transparent conductive oxide, TCO, is a problem as it limits the width of the segments. Another problem is that TCO-based glass is expensive, and the use of two TCO-based glasses in the DSC construction increases the cost even further. In order to resolve these problems, attempts have been made to exchange the TCO-based glass of the back contact by vacuum deposit of a porous conductive metal layer on the TiO2 by using metal sputtering techniques. Since the deposited sputtered porous metal layer is electrically conductive, the TCO-based glass can be exchanged with a TCO-less glass, which is much cheaper.
In Yohei Kashiwa, Yorikazu Yoshida, and Shuzi Hayase, PHYSICS LETTERS 92, 033308 (2008)) is described electro-spraying of a tetrapod-shaped ZnO onto the TiO2 layer followed by sputtering of titanium metal on top of the ZnO covered TiO2 layer. The tetrapod-shaped ZnO, which was embedded in the titanium layer, was then washed away by subsequent ZnO dissolution in HCL in order to form a sufficiently porous titanium layer. The porosity of the titanium layer must be sufficient in order not to create electrolyte ion diffusion limitations with resistive losses as a consequence. Also, the dye-sensitization process can be slowed down due to of diffusion problems through the titanium layer. Consequently, it was necessary to introduce pores in the sputtered titanium layer. The overall light-to-electric energy conversion efficiency obtained was 7.43%.
Yohei Kashiwa, Yorikazu Yoshida, and Shuzi Hayase, PHYSICS LETTERS 92, 033308 (2008)) and US2009314339 describe methods for increasing porosity of vacuum deposited metal layers. In US2009314339 a fine-particle layer is formed on the surface of the porous TiO2 layer and subsequently a conductive metal film is formed on the surface of the fine-particle layer; and thereafter the fine-particle layer is removed by heating or solvent-cleaning. A sputtered porous titanium layer deposited on top of a TiO2 layer is also disclosed in J. M. Kroonl, N. J. Bakker, H. J. P. Smit, P. Liska, K. R. Thampi, P. Wang, S. M. Zakeeruddin, M. Graetzel, A. Hinsch, S. Hore, U. Wu{umlaut over ( )}rfel, R. Sastrawan, J. R. Durrant, E. Palomares, H. Pettersson, T. Gruszecki, Walter, K. Skupien and G. E. Tull, Prog. Photovolt: Res. Appl. 2007; 15:1-18 (ENK6-CT2001-00575 NANOMAX).
The overall light-to-electric energy conversion efficiency obtained was 3.6%. These scientists concluded that further research was needed in order to improve efficiency.
Vacuum-based electron beam vapor deposition has been used to deposit a porous titanium layer on top of the TiO2 layer, Nobuhiro FUKE Japanese Journal of Applied Physics Vol. 46, No. 18, 2007, pp. L420-L422, Back Contact Dye-Sensitized Solar Cells vacuum process; Nobuhiro Fuke, Atsushi Fukui, Ryohichi Komiya, Ashraful Islam, Yasuo Chiba, Masatoshi Yanagida, Ryohsuke Yamanaka, and Liyuan Han, Chem. Mater. 2008, 20, 4974-4979. The overall light-to-electric energy conversion efficiency in these studies was between 7.1 and 8.4%.
Vacuum deposition of metal layers has several disadvantages:                Vacuum deposition is slow compared to other techniques, such as printing techniques.        Equipment used for vacuum deposition is relatively expensive.        Vacuum equipment requires substrates that do not give off gases under vacuum conditions.        Vacuum deposited porous metal layers have low permeability for ions in the DSC electrolyte.        Vacuum deposited porous metal layers have low permeability for dye-sensitization molecules resulting in longer dye-sensitization times.        Vacuum techniques require masking in order to deposit metal particles at the right place in the DSC.        Since deposited material is spread non-selectively on the surface the substrate in the deposition chamber, deposited metal material is wasted during deposition.        Metal targets used for vacuum deposition are expensive.        
Advantages with the vacuum process are that porous metal films with both good mechanical stability and good electrical conductivity can be formed. It is probable that the advantages are partly due to that the vacuum allows for the deposition of pure metal particles in an oxygen-free atmosphere. The absence of oxygen during deposition makes it possible to form good metallic particle-to-particle contact. The particle-to-particle contact is achieved due to the metal particles having high purity and being essentially free from metal oxide on the surface. During sputtering, the substrate is bombarded with high-energy metal particles. The large physical contact area increases the binding energy between the particles and the substrate, and the binding energy in the metal particle-to-particle contact, which results in a strong mechanical adhesion of the metal particles and the substrate and a strong mechanical particle-to-particle adhesion